Tuesday, October 7, 2014

What is a
Windows hash?

A Windows hash is a non-salted algorithmic encoding of a
plaintext password. Windows has used two
different algorithms for hashing to date, the result being an LAN Manager (LM) hash, or an NT hash. In a Microsoft Windows network, NT LAN
Manager (NTLM) is a suite of protocols used to provide authentication,
integrity and confidentiality to users.
NTLM is the successor to LAN Manager (LANMAN), and attempts to provide
backwards compatibility with LANMAN.

In the world of cryptography, salting is a known piece of
additional random data that is “mixed in” as an additional input to the one way
hashing function.The advantage of
salting is that the hashed representation of a password will never be the same
for the same plaintext string.Unfortunately this technique is not used with stored hashes in Windows.

The LANMAN hash is constructed as follows:

The plaintext password is padded with NULL characters
to a length of fourteen bytes, and converted to all uppercase.

The resulting string is split into two seven character
strings.Each string is used as a key to
encrypt the constant string value of “KGS!@#$%”. The data encryption
standard (DES) algorithm is used for encryption.

The resulting ciphertext is
concatenated into a sixteen byte hash value.

The NT hash is constructed by using the MD4 hashing algorithm
over the plaintext string. A plaintext
password, used to construct the NT hash, can be up to 256 characters in length
with mixed case and special characters.
Unlike the LM hash, special characters, upper and lower case are all
preserved with an NT hash.

How is a
Windows hash passed across the network?

From a network transmission perspective, there are two
different challenge response algorithms in use.The LANMAN, and NTLMv1 algorithm operates as
follows:

1)Workstation client initiates authentication

2)Server responds with a random challenge

3)Client formulates and sends a response to the challenge
by:

a)Padding the LANMAN/NT hash to 21 bytes

b)Splitting the hash into three seven character pieces

c)Using each piece as a DES key to encrypt the server
challenge.

NTLMv2 challenge/response is significantly more secure, and
operates as follows:

1)Workstation client initiates authentication

2)Server responds with a random challenge

3)Client formulates/sends a response by:

a)Creating an HMAC-MD5 string using the username, domain
name, and NT hash as a key.This result
is called the NTLMv2 one way function (OWF)

b)Combining the HMAC-MD5 string, server challenge,
timestamp, and client challenge into a response.

Obtaining
Hashes during a Penetration Test

In a post exploitation scenario, and assuming you have
administrative access, it is useful to be able to retrieve the hashed
representation of passwords. Windows hashed passwords are stored in the
Security Accounts Manager (SAM) registry hive which is a file named
“%SystemRoot%\System32\Config\SAM”.By
default, the SAM file is locked on a running system, and inaccessible to all
users including administrative users.If
an NT file system recovery was performed in the past, and the Administrator has
not removed the backup data, you might be able to find the SAM file in the
“%SystemRoot%\repair” directory.

Since you cannot read the file directly on the disk volume,
there are a few alternative tricks to get hold of the data.The local security authentication sub-system
process (LSASS.EXE) on a running windows system reads this data and caches it
in memory.

Method 1: Meterpreter “hashdump” command

The hashdump command is an in-memory version of “pwdump”.Hashdump allocates memory inside the LSASS
process, injects assembly code, and executes it using the CreateThread()
function.The injected assembly code is
designed to read the hashes out of LSASS memory, and print them out.No files are written to disk, thus detecting
the technique without using memory forensic techniques is difficult.

In January, 2010, HD Moore noted that Anti-Virus, and host
intrusion prevention (HIPS) vendors had developed techniques to detect the API
calls made by the meterpreter hashdump command, and block the calls.In the process of detection however, LSASS
will often crash leading to system instability.

Method 2: Meterpreter “hashdump” post module

The hashdump post module (post/windows/gather/hashdump.rb) uses
a registry based technique to directly access the SYSKEY, and decrypt the raw
LANMAN, and NT hashes.The
significant advantage of this technique is that there is no in-memory
manipulation of LSASS with its potential for instability.

The caveat is that a SYSTEM token is required to use the
technique.An account in the local
administrators group does not have read access to the SAM registry tree that
contains the hashes.If you have
exploited a system service, or perhaps something like a DCERPC vulnerability,
you are in great shape.Alternatively
you can consider migrating your meterpreter to a service process, or load the
incognito module, and impersonate a token.

Method 3: Meterpreter “smart_hashdump” post
module

Carlos Perez improved further on the hash extraction techniques
in the sense of automating some of the additional steps, and logic required
before the actual hashes are extracted.The “smart_hashdump” post module works as follows:

Determine the
privilege level, operating system, and test if the target is a domain
controller

If you have the
right privileges, specifically a SYSTEM token, smart_hashdump will read the
hashes from the SAM registry hive.

If the system is a
Domain Controller, smart_hashdump will always inject assembly code into LSASS
to obtain hashes.

If the target is a
Windows 2008 server, and the process has administrative privileges,
smart_hashdump will:

Attempt to gain
SYSTEM by using “getsystem”.

If it fails to get
system, smart_hashdump will attempt to automatically migrate to a process that
has system privileges.

Inject the LSASS
process with assembly code to dump the hashes

If the target is a
Windows 7+ system with UAC disabled and the process has administrative
privileges, smart_hashdump will run “getsystem” and use the registry reading
method.If the target is a
Windows 2000/2003/XP system, “getsystem” will be used followed by the registry
reading method.

The take home point
with “smart_hashdump” is that it will do the work for you to obtain SYSTEM
privilege and prefer reading the SAM registry hive if at all possible.The downside with this and all methods, is
that within the context of a Domain Controller, you are still required to
inject assembly code into LSASS in order to obtain hashes.As pointed out above, this is a dangerous
technique and can result in a crash which your client would certainly not
appreciate.

Method
4: Extract hashes from Volume Shadow Copies of the file system

In 2011, Tim Tomes, and Mark Baggett were performing research
on the topic of hiding malware in Volume Shadow Copies.As a result of this work, they realized that
by creating a volume shadow copy, or using a pre-existing volume shadow copy,
the NTDS.DIT, SAM and SYSTEM files could be directly copied from a running
system.

In summary, the process involves:

●Creating a volume shadow copy using system tool
“vssadmin” or Tim’s visual basic script called “Vssown.vbs”

The
largest single advantage with using an offline method to extract hashes after
copying from a volume shadow is the fact that you do not have to inject
anything into the LSASS process on a running domain controller.The disadvantage is that in a larger
environment, you are faced with exfiltrating some potentially large files, and
downloading them for analysis.It is
possible that network defenders might detect the network activity of a large
data transfer, and/or a large file being written to disk on a workstation.

Choosing the right tool for the job

Which method of hash extraction you decide to use depends on the context of
your test.If you are dealing with an
individual system that is not a domain controller, then the “smart_hashdump”
module is a good solid choice.If you
are working with host based IPS or Anti-Virus that is detecting LSASS injection
attempts, you are taking a significant risk when injecting assembly into this
process.In the context of a domain
controller, a volume shadow copy extraction is the best approach to ensure
stability.It is apparent that the
original LSASS injection process in meterpreter can be risky, and should
probably be avoided.

While this article
focuses on hashes, it would be remiss not to mention that LSASS also caches
some plaintext data for other security service providers.These include WDigest for HTTP
authentication, Kerberos, and Terminal Services.If you have administrative access on a local
system, LSASS plaintext security service provider memory can be accessed using
the Mimikatz or Kiwi extensions enabling the extraction of plaintext
information to screen.Kiwi can be
thought of as the upgraded (version 2) release of Mimikatz and includes some
excellent new functionality.